In certain conventional four-wheel drive vehicles (4WD vehicles), a transfer device is installed integrally with a transmission. The transfer device has a plurality of switching mechanisms such as a two-wheel/four-wheel switching mechanism for switching over between two-wheel drive (2WD) and four-wheel drive (4WD) and a high-speed/low-speed switching mechanism for switching over between high speed (H) and low speed (L). These switching mechanisms are operated with operation of each shifting shaft which is actuated, for example, by operation of a lever.
In the transfer device shown in FIG. 11, a high-speed/low-speed shifting shaft 304 with a high-speed/low-speed fork 302 fixed thereto and a two-wheel/four-wheel shifting shaft 308 with a two-wheel/four-wheel fork 306 fixed thereto are disposed side by side in proximity to each other. A high-speed/low-speed shifting groove 310 is formed in the outer peripheral surface of the high-speed/low-speed shifting shaft 304, while a two-wheel/four-wheel shifting groove 312 corresponding to the high-speed/low-speed shifting groove 310 is formed in the outer peripheral surface of the two-wheel/four-wheel shifting shaft 308. A shift lever 314 comes into engagement with the high-speed/low-speed shifting groove 310 and the two-wheel/four-wheel shifting groove 312 selectively by shift and select operations, whereby the vehicular running condition is switched over between two-wheel drive (2WD) and four-wheel drive (4WD) or between high speed (H) and low speed (L).
For conducting the switching operations by utilizing an actuator without going through the select operation of the lever, there has heretofore been adopted such a construction as shown in FIG. 12, wherein a shifting yoke 410 is fitted on both a high-speed/low-speed shifting shaft 404 with a high-speed/low-speed fork 402 fixed thereto and a two-wheel/four-wheel shifting shaft 408 with a two-wheel/four-wheel fork 406 fixed thereto. A high-speed/low-speed semicircular groove 412 is formed in the outer peripheral surface of the high-speed/low-speed shifting shaft 404, while a two-wheel/four-wheel semicircular groove 414 corresponding to the high-speed/low-speed semicircular groove 412 is formed in the outer peripheral surface of the two-wheel/four-wheel shifting shaft 408. Further, a ball 416 is disposed within the shifting yoke 410 and is brought into engagement selectively with the high-speed/low-speed semicircular groove 412 and the two-wheel/four-wheel semicircular groove 414. Though not shown, when the high-speed/low-speed shifting shaft 404 and the two-wheel/four-wheel shifting shaft 408 are actuated for switch-over to high-speed two-wheel drive (2H), high-speed four-wheel drive (4H), or low-speed four-wheel drive (4L), by means of an actuator, there are used two types of actuators which are an actuator for shift operation and an actuator for select operation, or a three-stop position actuator is used.
Transfer devices of such a construction are disclosed, for example, in Japanese Patent Laid-Open Nos. 10-53045 and 8-156625 and Japanese Utility Model Laid-Open No. 6-80053. According to the transfer device disclosed in Japanese Patent Laid-Open No. 10-53045, a two-wheel/four-wheel switching shaft member and a high-speed/low-speed switching shaft member are disposed coaxially. The two-wheel/four-wheel switching shaft member operates in its axial direction independently of the high-speed/low-speed switching shaft member to effect switching between two-wheel drive and four-wheel drive, whereby it is intended to attain a reduction of size. According to the transfer device disclosed in Japanese Patent Laid-Open No. 8-156625, three shift positions four-wheel high-speed position (4H), four-wheel high-speed direct-coupled position (4HLc), and four-wheel low-speed direct-coupled position (4LLc)--are switched over from one to another using a single switching rail, thereby making the safety for on-road use and running performance for off-road use compatible with each other. The transfer device disclosed in Japanese Utility Model No. 6-80053 intends to simplify the construction thereof by switching in interlock with a lever.
However, in such conventional transfer devices, as shown in FIG. 11, a select operation of a shift lever is needed at the time of switching between two-wheel drive (2WD) and four-wheel drive (4WD) or between high speed (H) and low speed (L) with use of the shift lever, and this operation is troublesome.
Further, where the vehicular running condition is to be switched over to high-speed two-wheel drive (2H), high-speed four-wheel drive (4H), or low-speed two-wheel drive (4L), using an actuator, it has so far been necessary to use two types of actuators or use a three-stop position actuator. These actuators enhance the accuracy of stop positions (4H), particularly the accuracy of the middle stop position out of three stop positions. However, such an accuracy enhancing operation is troublesome.